Speaker system having directivity

ABSTRACT

A speaker system comprises a pair of speakers for a higher frequency range and a pair of speakers for a lower frequency range. The speakers for the lower range are disposed with a space d 1  equal to the wavelength λ c  of a division frequency f c  of a reproduction frequency range, and speakers for the higher range are disposed with a space d 2  =d 1  /4 to d 1  /2 in the middle of the lower frequency speakers. Another speaker system comprises a speaker for reproducing signals in a lower frequency range of a reproduction frequency range to be reproduced, and a speaker for reproducing a higher frequency range of the frequency range, wherein a sound path dividing construction is provided for dividing the sound path of each of the speakers into two paths to provide openings for the lower frequency range and the higher frequency range. The openings for the lower frequency range are disposed with a space d 1  therebetween while the openings for the higher frequency range are disposed with a space d 2  therebetween, where d 1  =λ c  ±50% and d.sub. 2 =d 1  /4 to d 1  /2.

BACKGROUND OF THE INVENTION

This invention relates to a loudspeaker system effecting sounddirectivity, in which a plurality of speakers are disposed in a suitablyspaced relation for causing a strong sound pressure in a specificdirection by mutual interference of acoustic waves.

Conventionally, a loudspeaker system of a Tonsaule type in which aplurality of speakers, e.g., four speakers, are arranged in a straightline as shown in FIG. 6, have been well known as a speaker system foreffecting sound directivity.

With the Tonsaule type speaker system as shown in FIG. 6, the speakersSP are aligned in a line with a predetermined distance d between eachand other speakers. Thus, as shown in FIG. 3, the sound pressures fromtwo speakers SP cancel each other out to produce a zero pressure at apoint P₉₀ in a direction of 90° relative to a point P₀ on the centeraxis, such cancellation being due to the fact that the phase differencebetween the two sound pressures is 180° out of phase at the point P₉₀ ata frequency f₀ and having wavelength of which is given by d=λ/2.

At the point P₀, the sound pressures strengthen each other to cause apeak value of the sound pressure in the P₀ direction. At a point P.sub.θbetween the point P₀ and the point P₉₀ the sound pressure is of a valuebetween the peak value and the zero pressure, with the sound pressuredecreasing gradually toward zero pressure with increasing angles ofdirection toward P₉₀, thus resulting in a directive pattern indicated bya solid line curve as shown in FIG. 4.

However, with sound at a frequency having a wavelength d=λ, the soundpressures strengthen each other at the point P₉₀, causing a directivepattern as shown by a dotted line in FIG. 4.

For the reasons described above, the Tonsaule type speaker systemexhibits, as shown in FIG. 7, a directivity pattern depicted by a solidline A at the mid frequency range and by a dotted line B at lowerfrequency range, thus causing or effecting an inadequate sounddirectivity.

Further, the Tonsaule arrangement is further disadvantaged in that it isrequired that the speakers be disposed or distanced in a spaced relationof d=λ/2, which leads to a large overall size of the system.

In addition to the Tonsaule type speaker, a parametric speaker has alsobeen in practical use but it has not been widely used because it suffersfrom problems in that an ultrasonic modulation device is required, andreproduction of the sound at the lower frequency range is inherentlydifficult.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a loudspeaker system inwhich the drawbacks of the conventional speaker systems consisting of aplurality of speakers are eliminated, a strong sound directivity isobtained in the direction of the central axis, and the system may beconstructed to be compact in size.

A speaker system according to the present invention is of a type inwhich the frequency range to be reproduced is divided at an arbitraryfrequency or division frequency into a higher frequency range and alower frequency range, each of which being reproduced through acorresponding pair of speakers. The low speakers for the lower range aredisposed with a space d₁ equal to the wavelength of the divisionfrequency therebetween, and the high speakers for the higher range aredisposed in the middle of the lower frequency speakers with a space d₂=d₁ /4 to d₁ /2 therebetween.

Another speaker system according to the invention comprises a speakerfor reproducing signals in the lower frequency range and a speaker forreproducing signals in the higher frequency range, and a sound pathdividing means for dividing the sound path of each of the speakers intotwo paths to provide openings for the lower frequency range and thehigher frequency range. The openings for the lower frequency range aredisposed with a space d₁ therebetween, while the openings for the higherfrequency range are disposed with a space d₂ therebetween, where d₁=λ_(c) ±50% and d₂ =d₁ /4 to d₁ /2 (λ_(c) being a wavelengthcorresponding to a division frequency).

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and objects of the present invention will be moreapparent from the following description of preferred embodiments and theaccompanying drawings in which:

FIG. 1 is an illustrative drawing of an embodiment of the presentinvention;

FIG. 2 is a diagram for illustrating the directivity pattern of thepresent invention;

FIG. 3 is a diagram for showing a sound pressure produced by twospeakers;

FIG. 4 shows the directivity pattern obtained from the configuration ofFIG. 3;

FIG. 5 is a diagram for showing another embodiment of the invention;

FIG. 6 is a diagram for illustrating the principle of a conventionalTonsaule type speaker system; and

FIG. 7 is a diagram for illustrating the directivity pattern obtainedfrom the configuration of FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the invention will now be described withreference to the drawings.

In FIG. 1, a pair of low frequency range speakers (low-range speakers),Ll and Lr, are disposed side by side, and supplied signals through a lowpass filter (not shown). The low pass filter attenuates the signal levelin such a way that the sound pressure is decreased by 6 dB at a divisionfrequency f_(c) (at which the entire reproduction frequency range isdivided into a high frequency range and a low frequency range), anddecreased by 18 dB at a frequency 2f_(c). The low range speakers aredisposed with a space d₁, which is equal to a wavelength λ_(c)corresponding to the division frequency f_(c).

A pair of high frequency range speakers (high-range speakers), Hl andHr, are disposed with a space d₂ =d₁ /4 between or in the middle of theaforementioned low-range speakers Ll and Lr. Signals are supplied to thehigh-range speakers Hl and Hr through a high pass filter (not shown)which attenuates the signal level in such a way that the sound pressureis decreased by 6 dB at the frequency f_(c) and by 18 dB frequency f_(c)/2. Each of the high and low frequency range speakers has the samevolume velocity, and each can be thought of as a separate point orsource of sound.

In the direction of 90° relative to the central axis, the resultantsound pressure P_(t) (at the point P₉₀ sufficiently remote from thepoint source by a distance r) is given by the following equation:##EQU1## where k is the wavelength (=ω/c=2πf/c);

c is the velocity of sound;

G₁ is the gain of the low frequency range filter;

α is the phase of the low frequency range filter;

G₂ is the gain of the high frequency range filter; and β is the phase ofthe high frequency range filter.

Thus since G₁ ≃1, α≃0, and G₂ ≃-18 dB (=0.125) at a frequency of f₁=f_(c) /2, the sound pressure is given by: ##EQU2##

Since G₁ ≃0.125, G₂ ≃1, and β≃0, at a frequency f₂ =2f_(c) =4f₁, thesound pressure will be exactly the same as for equation (2) except thatthe sound pressure differ in phase at higher frequencies. That is, thesound pressure is given by: ##EQU3##

Since G₁ =0.5, α≃-π/2, G₂ =0.5, and β=π/2 at a frequency f_(c) betweenf₁ and f₂, if the signal is applied in a reversed polarity to the highfrequency filter, then one can obtain β=-π/2.

Thus the sound pressure is given as follows: ##EQU4##

Here f_(c) =2f₁ =f₂ /2, therefore d₁ =λ_(c), d₂ =d₁ /4, where λ_(c) isthe wavelength of f_(c).

Thus one can obtain:

    cos (kd.sub.1 /2)=cos (π)=-1

    cos (kd.sub.2 /2)=cos (π/4)=0.707,

and further, one can obtain: ##EQU5##

Moreover, the sound pressure P_(t0) at the point P₀ on the central axisis given by: ##EQU6##

The ratios R's of the sound pressure in the direction of 90° relative tothe central axis, to that at a point on the central axis, will be asfollows at the frequencies f1, f2, and fc, respectively ##EQU7## thusresulting in an atte uat on of he sound pressure of more than 20 dB withrespect to the sound pressure P_(t0) on the central axis.

The above result is the case where it is assumed that the speakers haveno inherent directivity at all. As a practical matter, a still largerattenuation can be expected because actual speakers inherently have somedirectivity.

Further, at angles from 0° to 90°, the phase difference between thesound pressures due to the different distances from the individualspeakers will be within π/2 within the respective frequency range.Accordingly, there will not be effected peak values in the soundpressure distribution, but there will be an attentuation effect due tocancellation of the sound pressure. Thus, an ideal directivitycharacteristic in which the sound pressure decreases smoothly can beobtained as shown in FIG. 2.

The embodiment thus far described is a basic configuration of theinvention in which a set of four speakers are used to obtain a narrowdirectivity over a frequency range of two octaves ranging from f₁ to f₂.An extended frequency range of two additional octaves can be obtained onthe high frequency side and/or the low frequency side by disposing twoadditional speakers for the high frequency range and/or the lowfrequency range in the same manner as described previously, i.e., with aspace of 1/4 of d₂ for the higher frequency range and a space four timesd₁ for the lower frequency range, respectively.

Advantageously, the individual speakers to be used should be of a strongdirectivity; thus a horn type speaker or the addition of a horn baffleto a cone type speaker will provide a further improved directivity. Forexample, as shown in FIG. 5, two driving speakers are used, one HF forthe high frequency range and the other LF for the lower frequency range,and with the sound path of each speaker being divided by a horn 10 intotwo paths to provide horn openings at two locations, i.e., openings 20for the higher frequency range and openings 30 for the lower frequencyrange, thereby providing the same effect as the previously mentionedfour speakers arrangement. Additionally, the space d₂ between the highfrequency openings 20 and the space d₁ between the low frequencyopenings 30 are arranged in a manner similar to the embodiment shown inFIG. 1.

While the basic embodiment has been described with the highestattentuation obtained in the direction of 90° with respect to thecentral axis, a directivity characteristic having a further narrowerangle can be implemented. Such can be effected by setting the spaces d₁and d₂ in such a way that distances d₁ ' and d₂ ' for an angle θ lessthan 90° relative to the central axis are λ/2, respectively, as shown inFIG. 1. The distances d₁ ' and d₂ ' are given by the followingequations, respectively:

    d.sub.1 '=d.sub.1 sin θ

    d.sub.2 '=d.sub.2 sin θ

For example, if the angle θ=45°, then the sound pressure theoreticallyis supposed to become a maximum again in the direction of 90°. However,in practice the inherent directivity of the speakers comes into play toactually provide a considerable net attentuation in the 90° directiondependent on the diameters and the frequency, and thus the resultantsound directivity will still be narrow.

It is not necessary that the spaces d₁ and d₂ be strictly maintained toλ_(c) and d₁ /4, respectively, because the inherent directivity of thespeakers and a diffraction effect due to the practical effect thatgeometry of the baffle or the cabinet (to which the speakers aremounted) influence the overall directivity characteristic. Thusconditions that deviate somewhat from the above conditions may wellexhibit a better directivity characteristic. However, experiments haverevealed that the directivity characteristic will be degraded beyond thefollowing conditions: ##EQU8##

The present invention can provide a narrow directivity over thefrequency range of two octaves by using four speakers and even widerfrequency range by using additional speakers, and can provide a smoothattenuation of the sound pressure with an increase in angle from thecentral axis without side lobes developing in its directivity pattern.

The speaker system according to the present invention can be made smallin size as compared to a conventional Tonsaule type speaker system, anda wider reproduction frequency range for the same directivity may beobtained with an increment of two octaves for each successive additionof a pair of speakers.

What is claimed is:
 1. A speaker system for effecting sound directivity,said speaker system comprising two pairs of speakers for reproducingsignals in a frequency range, said frequency range being divided at afrequency f_(c) having a wavelength of λ_(c) into a higher frequencyrange and a lower frequency range, a first pair of said two pairs ofspeakers reproducing signals in said higher frequency range and a secondpair of said two pairs of speakers reproducing signals in said lowerfrequency range, whereinsaid second pair of speakers for the lowerfrequency range is disposed with a first space d₁ therebetween and saidfirst pair of speakers for the higher frequency range is disposed with asecond space d₂ therebetween in such a way that

    d.sub.1 =λ.sub.c ±50% and d.sub.2 =d.sub.1 /4 to d.sub.1 /2.


2. A speaker system for effecting sound directivity, comprising:alow-range speaker for reproducing signals in a lower frequency range ofa reproduction frequency range divided by a division frequency f_(c)having a wavelength λ_(c), a high-range speaker for reproducing signalsin a higher frequency range of the reproduction frequency range dividedby the division frequency f_(c), and sound path division means fordividing a sound path of each of said low-range and high-range speakersinto two paths to provide openings for said lower frequency range andopenings for said higher frequency range, wherein said openings for saidlower frequency range are disposed with a space d₁ therebetween and saidopenings for said higher frequency range are disposed with a space d₂therebetween, in such a way that d₁ =λ_(c)±50 % and d₂ =d₁ /4 to d₁ /2.